1. Field of the Invention
This invention relates to integrated circuit chips and, more particularly, to techniques for setting the operating voltage of integrated circuit chips to improve speed and power performance.
2. Description of the Related Art
There is growing demand to operate integrated circuit chips at higher and higher speeds. However, higher speed operation generally increases the chip's power dissipation. Both power dissipation and maximum operating speed are related to the operating voltage supplied to the chip. Increasing the supply voltage enables a chip to operate at a higher speed, but also causes higher power dissipation. Conversely, decreasing the supply voltage decreases the maximum operating speed, but also reduces the power dissipation.
Typically, integrated circuits may be sorted during testing according to broad categories of maximum operating speed at a nominal operating voltage. For example, processor chips may be sorted into bins of 1.2 GHz, 1.0 GHz and 0.8 GHz at a nominal 1.3 volts. During testing a processor may be operated at 1.3 volts while maximum operating speed is determined, dependent on the measured power dissipation. The processor is then placed in a bin according to the highest speed at which the processor operates with power dissipation below a predetermined threshold value.
Generally speaking, demand is highest for the highest maximum operating speed integrated circuits. Furthermore, there are only a few values of maximum operating speed for a given integrated circuit since each additional speed value adds to the expense and complexity of designing, manufacturing, testing, marketing, and maintaining a parts inventory for equipment that uses the integrated circuit. Circuit applications also place constraints on the acceptable power dissipation. Consequently, a single power dissipation threshold is generally used during integrated circuit testing.
In addition to the above considerations, decreasing operating voltage may cause power dissipation to decrease faster than the rate at which maximum operating speed decreases. Consequently, for some number of integrated circuits that operate at an acceptable speed but at a power level that is slightly above the power threshold, it may be possible to choose a lower operating voltage at which the power dissipation drops below the power threshold while an acceptable maximum speed is maintained. Unfortunately, even though test equipment may be capable of determining an optimum supply voltage for each integrated circuit, changing the supply voltage on a part-by-part basis is problematic. For instance, using a different voltage in each circuit application adds cost and complexity to the hardware. Variable voltage supplies are more costly and complicated than fixed supplies. For a circuit application to deliver the desired voltage to a particular integrated circuit using a variable supply, some method of identifying the desired operating voltage for that particular integrated circuit would be needed. It would be desirable to set the operating voltage on a per-chip basis without increasing the complexity of the application circuitry. It would also be desirable to set the operating voltage on a per-chip basis without increasing the size and/or number of pins of the integrated circuit itself. In addition, it would be desirable to be able to use integrated circuits that operate at a voltage other than the nominal voltage in existing hardware designs.